This invention relates in general to a subambient pressure coolant loop for a fuel cell power plant, and deals more particularly with a subambient pressure coolant loop for a fuel cell power plant wherein the parasitic energy required to maintain the reactants of the fuel cell power plant within operating parameters is substantially reduced.
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a reactant fuel being provided to an anode electrode and a reactant oxidant being provided to a cathode electrode, generating an external current flow there-between. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Typically, electrochemical fuel cell assemblies employ a hydrogen-rich gas stream as a fuel and an oxygen-rich gas stream as an oxidant, whereby the resultant reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrode substrates formed of porous, electrically conductive sheet materialxe2x80x94typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION(trademark) and well known in the art. Catalyst layers are formed between the PEM and each electrode substrate to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit connected to a load to the cathode. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers may be represented by the following equations:
Anode reaction: H2xe2x86x922H++2e
Cathode reaction: xc2xdO2 30 2H++2exe2x86x92H2O
In practical applications, a plurality of planar fuel cell assemblies are typically arranged in a stack, commonly referred to as a cell stack assembly. The cell stack assembly may be surrounded by an electrically insulating housing that defines the various manifolds necessary for directing the flow of a hydrogen-rich fuel and an oxygen-rich oxidant into and out of the individual fuel cell assemblies, as well as a coolant stream, in a manner well known in the art. The cell stack assembly, including any associated components such as a degasifier, a demineralizer, a steam reformer, a heat exchanger and the like may, as a whole, be referred to as a fuel cell power plant.
As will be appreciated by one so skilled in the art, tying these differing components into a cohesive fuel cell power plant operating within specific design parameters results in a complex and oftentimes cumbersome structure.
Specifically, in the operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode, including water resulting from proton drag through the PEM electrolyte, and rates at which water is removed from the cathode or supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain an optimal water balance as the electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out, thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM thus decreasing cell performance.
A common approach to maintaining appropriate water equilibrium within a fuel cell power plant resides in a judicious balancing of pressures between the fuel and oxidant reactants and the coolant stream. By providing the fuel and oxidant streams at a higher pressure than that of the coolant stream, the flow of excess water created within the fuel cell assembly may be assuredly directed towards the coolant stream for removal and subsequent resupply.
An example of a fuel cell power plant utilizing such a balancing of pressures is shown in commonly owned U.S. Pat. No. 4,769,297 issued to Reiser et al., on Sep. 6, 1988, and incorporated herein by reference in its entirety. The water management system of Reiser utilizes entrained water in the hydrogen fuel gas stream provided to the anode electrode of each fuel cell as a coolant to help humidify and cool the fuel cell power plant during operation. The hydrogen fuel and entrained water will humidify the anode electrode, while the proton dragging effect will cause excess water to migrate across the ion exchange membrane to the cathode electrode, thereby also humidifying the ion exchange membrane. Reiser maintains a pressure differential between the anode flow field and the cathode flow field, both of which are maintained above ambient pressure, such that water accumulating at the cathode electrode of each fuel cell migrates across a separator plate to the next anode electrode in the cell stack assembly of the fuel cell power plant. The water management system of Reiser therefore does not utilize a separate coolant loop.
Another fuel cell power plant operating system is disclosed in commonly owned U.S. Pat. No. 5,064,732 issued to Meyer on Nov. 12, 1991, and is incorporated herein by reference in its entirety. The operating system of Meyer is also devoid of a dedicated coolant loop and instead utilizes a pressure differential between the anode and cathode flows of the fuel cell to assist in the proper migration of water throughout the fuel cell power plant. In Meyer, a back pressure regulator is utilized with a porous element in order to remove excess water from the anode of the fuel cell power plant.
Another fuel cell power plant operating system is disclosed in commonly owned U.S. Pat. No. 5,503,944 issued to Meyer et al. on Apr. 2, 1996, and is incorporated herein by reference in its entirety. The operating system of Meyer et al. incorporates a dedicated coolant loop while also utilizing a pressure differential between the reactant gasses and the coolant to assist in the proper migration of water throughout the fuel cell power plant. In Meyer et al., the coolant loop may be operated at approximately ambient pressures provided that the pressure of the oxidant reactant is slightly above that of the coolant loop.
Another fuel cell power plant operating system is disclosed in commonly owned U.S. Pat. No. 5,700,595 issued to Reiser on Dec. 23, 1997, and is incorporated herein by reference in its entirety. The operating system of Reiser also utilizes a pressure differential between the reactant gasses and the coolant to assist in the proper migration of water throughout the fuel cell power plant. In Reiser, fine pore separator plates may be utilized throughout the fuel cell power plant due to this pressure differential.
Although each of the foregoing systems ensure the efficient migration of water throughout a fuel cell power plant by maintaining a relative pressure differential between the reactant gas streams and the coolant stream, the parasitic power requirements of these systems varies widely and affects the efficiency of the fuel cell power plants as a whole.
The present invention therefore seeks to reduce this parasitic power drain on the overall operating system of a fuel cell power plant by operating the coolant stream at a subambient pressure which in turn reduces the parasite power to pump the air. As it requires less power to pressurize a water stream in comparison to a gaseous stream, the present invention proposes to utilize this relationship in order to promote greater energy savings during operation of the fuel cell power plant.
With the forgoing problems and concerns in mind, the present invention seeks to provide a fuel cell power plant in which a multitude of separate components are integrated in order to allow the fuel cell power plant to operate at peak efficiency.
It is an object of the present invention to utilize a subambient pressure coolant loop for a fuel cell power plant.
It is another object of the present invention to utilize a subambient pressure coolant loop for a fuel cell power plant, and thereby reduce the parasitic energy required to maintain the reactants of the fuel cell power plant within operating parameters.
It is another object of the present invention to reduce the complexity of the fuel cell power plant by reducing the number of components needed to operate the fuel cell power plant.
It is another object of the present invention to allow reactant gases to be utilized at approximately ambient pressures.
According to one embodiment of the present invention, a fuel cell power plant having a fuel cell assembly includes an anode provided with a fuel stream, a cathode provided with an oxidant stream, an ion exchange membrane oriented between said anode and said cathode, and a coolant loop circulating a coolant stream in fluid communication with the fuel cell assembly.
An oxidant source is utilized for supplying the fuel cell assembly with the oxidant stream at a predetermined pressure, while a coolant regulator oriented along the coolant loop lowers the coolant stream to a subambient pressure prior to the coolant stream coming into fluid communication with the fuel cell assembly.
These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole.